The present invention relates to an optical deflection device which forms a scanning beam in an optical scanning unit which conducts scanning with a laser beam. The optical deflection device of the invention is used for a copying machine, a facsimile machine, a printer and a scanner, while, an image forming apparatus related to the invention is used for a copying machine, a facsimile machine, and a printer.
As a conventional optical scanning unit, those wherein a scanning laser beam is formed by rotating a rotary polygon mirror at high speed. As a motor which drives the rotary polygon mirror, those having a rotary body supported by ball bearings and those having a rotary body supported by dynamic pressure bearing devices each being called an air bearing, are available. The latter is attracting public attention as a powerful means to realize high speed copying machines and high speed printers, because it has efficiency to rotate at superhigh speed such as tens of thousands rpm.
Since the rotary polygon mirror rotates at high speed in optical scanning of a laser beam, there is a problem of noise caused by rotation at high speed. In the conventional optical deflection device, it has been found that a vibration is caused between a coil supporting member and a supporting base board when a rotary body is rotated, because the coil supporting member which supports a coil that rotates a rotary body has been fixed directly on the supporting base board with a fixing means such as a screw, and thereby a noise called an electromagnetic force noise is caused. Experiments have shown that the greater cause of the noise is the electromagnetic force noise, though a mirror surface causes a noise when it rotates through the air in the course of rotation of the rotary polygon mirror.
In an image recording apparatus such as a laser printer, a laser beam is caused to enter a polygon mirror of an optical deflection device rotating at high speed based on information obtained through reading as a writing means for images, then, the reflected light conducts scanning to be projected on the surface of a photoreceptor for image recording. FIG. 12 is a perspective view showing an embodiment of a beam scanning optical apparatus employing an optical deflection device composed of a polygon mirror.
In the drawing, the numeral 80 represents a semiconductor laser, 81 represents a collimator lens representing a beam forming optical system, 82 represents a first cylindrical lens, 83 is a polygon mirror, each of 84A and 84B is an f.theta. lens, 85 is a second cylindrical lens, 86 is a mirror, 87 is a cover glass, and 88 is a photoreceptor drum. Incidentally, the numeral 89 is an index mirror for detecting synchronization, 89S is an index sensor for sensing synchronization, and 83M is a rotation driving section for polygon mirror 83 of an optical deflection device.
A beam emitted from the semiconductor laser 80 is changed to a collimated light by the collimator lens 81, then, passes through the first cylindrical lens 82 of a first image forming optical system, and enters a mirror surface of the polygon mirror 83 which is rotating at high speed in uniform velocity. Then, the light reflected on the mirror surface is transmitted through a second image forming system composed of f.theta. lenses 84A, 84B, and the second cylindrical lens 85, and goes through mirror 86 and cover glass 87 to conduct (main) scanning on the circumferential surface of photoreceptor drum 88 with the prescribed spot diameter. Fine adjustment of the direction of the main scanning is made by an unillustrated adjusting mechanism, and detection of synchronization for each line is conducted by making a beam before the start of scanning to enter the index sensor 89S through the index mirror 89.
It is necessary, for obtaining an excellent latent image on photoreceptor drum 88 by the above-mentioned beam scanning optical unit, that a polygon mirror which rotates at high speed is made to be a polygon mirror forming a precise regular polygon, and it is free from eccentricity and inclination to the rotation axis, and it rotates without positional deviation in the axial direction.
A rotary mirror unit is provided thereon with a polygon mirror and a torque-generating permanent magnet which faces a coil provided on a printed board on the fixed base board, and it is arranged to rotate at high speed between an outer casing bearing of the mirror unit and a fixed inner cylinder bearing.
In the structure of a mirror unit which rotates on an optical deflection device, a cylinder portion of a flange having a disk portion which serves as a reference plane when a polygon mirror is mounted is put together solidly through the means of shrinkage fitting of an outer casing bearing, and the polygon mirror is sandwiched between a disk portion of a flange and a mirror holding plate on which a permanent magnet is fixed.
It was observed by the inventors of the invention that a cylindrical outer casing bearing of a mirror unit and a flange fitted with the outer casing bearing are in the relationship of stress, and the outer casing bearing is strained by fluctuations of ambient temperature to be in a tapered conical shape. When the outer casing bearing is strained, efficiency of the mirror unit which rotates on an inner cylinder bearing through an air bearing turns out to be unstable.
As a method to unite the cylinder portion of the flange solidly with the outer casing bearing, there also is a method, for example, to inject and fix an adhesive agent in a joining section between the outer casing bearing and the cylinder portion. In this method, however, it is necessary that adhesive agents are filled and hardened in the clearance which is uniform in terms of a gap for the entire section in the radial direction between the outer casing bearing and the cylinder portion. However, the greatest possible care is required to attain the state of joining, while satisfying the conditions stated above.
When joining the outer casing bearing and the cylinder portion with a means other than an adhesive agent, there exists an interacting stress between the outer casing bearing and the cylinder portion, and the relationship of the stress varies depending on fluctuations of ambient temperature, thus, the state of generated strain sometimes causes a possibility of relative deviation in the axial direction between the outer casing bearing and the cylinder portion.